This chapter considers the neurodegenerative disorders Alzheimer’s disease (AD), Lewy body dementias (dementia with Lewy bodies (DLB) and Parkinson’s disease dementia(PDD)), frontotemporal dementia (FTD); and also vascular dementia (VaD) which results from cerebrovascular disease. These different conditions, which give rise to dementia syndromes, each have distinct neurochemical pathologies, with important implications for treatment. As increased age is the common risk factor generally associated with dementing illnesses, neurochemical changes are set in the context of the changes which occur during ageing. A detailed understanding of the neurotransmitter function in each condition can lead to rational drug design and treatment strategies appropriate for each group of patients. Neurochemical pathology in transmitter systems underlying clinical features of these disorders are reviewed.